Resistance welding a porous metal layer to a metal substrate

ABSTRACT

An apparatus and method are provided for manufacturing an orthopedic prosthesis by resistance welding a porous metal layer of the orthopedic prosthesis onto an underlying metal substrate of the orthopedic prosthesis. The resistance welding process involves directing an electrical current through the porous layer and the substrate, which dissipates as heat to cause softening and/or melting of the materials, especially along the interface between the porous layer and the substrate. The softened and/or melted materials undergo metallurgical bonding at points of contact between the porous layer and the substrate to fixedly secure the porous layer onto the substrate.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 13/300,151, filed Nov. 18, 2011, which claims priority to U.S.Provisional Patent Application Ser. No. 61/414,978, filed Nov. 18, 2010,the disclosures of which are hereby expressly incorporated by referenceherein in its respective entirety.

FIELD OF THE DISCLOSURE

The present disclosure relates to a method of manufacturing anorthopedic prosthesis. More particularly, the present disclosure relatesto a method of manufacturing an orthopedic prosthesis having a porousmetal layer and an underlying metal substrate.

BACKGROUND OF THE DISCLOSURE

Orthopaedic prostheses are commonly used to replace at least a portionof a patient's joint to restore or increase the use of the jointfollowing traumatic injury or deterioration due to aging, illness, ordisease, for example.

To enhance the fixation between an orthopedic prosthesis and a patient'sbone, the orthopedic prosthesis may be provided with a porous metallayer. The porous metal layer may define at least a portion of thebone-contacting surface of the prosthesis to encourage bone growthand/or soft tissue growth into the prostheses. The porous metal layermay be coupled to an underlying metal substrate.

SUMMARY

The present disclosure provides an apparatus and method formanufacturing an orthopedic prosthesis by resistance welding a porousmetal layer of the orthopedic prosthesis onto an underlying metalsubstrate of the orthopedic prosthesis. The resistance welding processinvolves directing an electrical current through the porous layer andthe substrate, which dissipates as localized heat to cause softeningand/or melting of the materials, especially at points of contact alongthe interface between the porous layer and the substrate. The softenedand/or melted materials undergo metallurgical bonding at the points ofcontact between the porous layer and the substrate to fixedly secure theporous layer onto the substrate.

According to an embodiment of the present disclosure, a method isprovided for manufacturing an orthopedic prosthesis. The method includesthe steps of: providing a metal substrate; providing a porous metallayer having a thickness; positioning the porous layer against thesubstrate to form an interface between the porous layer and thesubstrate; and directing an electrical current to the interface betweenthe porous layer and the substrate to bond the porous layer to thesubstrate while maintaining the thickness of the porous layer.

According to another embodiment of the present disclosure, a method isprovided for manufacturing an orthopedic prosthesis having a metalsubstrate and a porous metal layer. The method includes the steps of:positioning the porous layer against the substrate to form an interfacebetween the porous layer and the substrate; and directing a pulsedelectrical current to the interface between the porous layer and thesubstrate to bond the porous layer to the substrate, the pulsedelectrical current including at least a first pulse and a second pulseseparated from the first pulse by a cooling time.

According to yet another embodiment of the present disclosure, a methodis provided for manufacturing an orthopedic prosthesis. The methodincludes the steps of: providing a metal substrate; providing a porousmetal layer having a net surface; positioning the net surface of theporous layer against the substrate to form an interface between theporous layer and the substrate; and directing an electrical current tothe interface between the porous layer and the substrate to bond theporous layer to the substrate. The net surface of the porous layer isformed by: providing a porous structure having an outer surface; coatingthe outer surface of the porous structure with metal to produce theporous layer; and after the coating step, maintaining the outer surfacewithout machining the outer surface to arrive at the net surface.

According to still yet another embodiment of the present disclosure, anapparatus is provided for manufacturing an orthopedic prosthesis havinga metal substrate and a porous metal layer. The apparatus includes ahousing defining a chamber with a controlled atmosphere, the chambersized to receive the orthopedic prosthesis, a controller, a powersource, and an electrode configured to establish electricalcommunication between the power source and the orthopedic prosthesis,the controller directing a pulsed electrical current from the powersource to the orthopedic prosthesis to bond the porous layer to thesubstrate.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned and other features and advantages of thisdisclosure, and the manner of attaining them, will become more apparentand the invention itself will be better understood by reference to thefollowing description of embodiments of the invention taken inconjunction with the accompanying drawings, wherein:

FIG. 1 is an elevational view of a prosthetic proximal femoralcomponent, the proximal femoral component including a porous metal layercoupled to an underlying metal substrate;

FIG. 2 is a cross-sectional view of the proximal femoral component ofFIG. 1;

FIG. 3 is a front elevational view of an exemplary apparatus used toassemble the proximal femoral component of FIG. 1,

FIG. 4A is a schematic diagram of the apparatus of FIG. 3, the apparatusincluding fixtures and weld heads that are shown in an open position toreceive the proximal femoral component;

FIG. 4B is a schematic diagram similar to FIG. 4A, the fixtures and theweld heads of the apparatus shown in a closed position to hold theporous metal layer against the metal substrate of the proximal femoralcomponent;

FIG. 5 is a graphical depiction of the average bond strength betweenvarious porous layers and metal substrates in accordance with Example#1;

FIG. 6 is another graphical depiction of the average bond strengthbetween various porous layers and metal substrates in accordance withExample #2;

FIG. 7 is another graphical depiction of the bond strength betweenvarious porous layers and metal substrates in accordance with Example#3;

FIG. 8 is a graphical depiction of the tantalum concentration gradientin a diffusion bonded sample and in a resistance welded sample;

FIG. 9 is a graphical depiction of the titanium concentration gradientin a diffusion bonded sample and in a resistance welded sample;

FIG. 10 is a scanning electron microscope image taken along theinterface between a porous component and a substrate of a diffusionbonded sample;

FIG. 11 is a scanning electron microscope image taken along theinterface between a porous component and a substrate of a resistancewelded sample;

FIG. 12 is another graphical depiction of the bond strength betweenvarious porous layers and metal substrates in accordance with Example#6;

FIG. 13 is a scanning electron microscope image taken along theinterface between a porous layer and metal substrate in accordance withExample #6;

FIG. 14 is a scanning electron microscope image taken along theinterface between a porous layer and metal substrate in accordance withExample #7; and

FIG. 15 is another graphical depiction of the bond strength betweenvarious porous layers and metal substrates in accordance with Example#7.

FIG. 16 is a microscopic image of a control sample of a porous metal.

FIG. 17 is a microscopic image of the same type and grade of porousmetal as in FIG. 16 except that during a welding step the porous metalwas contacted by an electrode having a textured surface.

FIG. 18 is a microscopic image of the same type and grade of porousmetal as in FIG. 16 except that during a welding step the porous metalwas contacted by an electrode having a non-textured surface.

FIG. 19 is a perspective view of an electrode in accordance with oneembodiment of the present disclosure.

Corresponding reference characters indicate corresponding partsthroughout the several views. The exemplifications set out hereinillustrate exemplary embodiments of the invention and suchexemplifications are not to be construed as limiting the scope of theinvention in any manner.

DETAILED DESCRIPTION

Referring to FIGS. 1 and 2, an orthopedic prosthesis is provided in theform of a proximal femoral component 10 (e.g., a hip stem). While theorthopedic prosthesis is described and depicted herein in the form of aproximal femoral component 10, the orthopedic prosthesis may also be inthe form of a distal femoral component, a tibial component, anacetabular component, or a humeral component, for example.

Proximal femoral component 10 of FIG. 1 includes stem 12 and neck 14,which is configured to receive a modular head (not shown). It is alsowithin the scope of the present disclosure that the head may beintegrally coupled to neck 14. In use, with stem 12 of proximal femoralcomponent 10 implanted into the intramedullary canal of a patient'sproximal femur, neck 14 and the head (not shown) of proximal femoralcomponent 10 extend medially from the patient's proximal femur toarticulate with the patient's natural acetabulum or a prostheticacetabular component. Stem 12 of proximal femoral component 10 includesan exterior, bone-contacting surface 18 that is configured to contactbone and/or soft tissue of the patient's femur.

As shown in FIG. 2, proximal femoral component 10 includes a metalsubstrate 20 and a porous metal layer 22 coupled to the underlyingsubstrate 20. Porous layer 22 may be disposed within recess 26 ofsubstrate 20. With porous layer 22 defining at least a portion ofbone-contacting surface 18, bone and/or soft tissue of the patient'sfemur may grow into porous layer 22 over time to enhance the fixation(i.e., osseointegration) between proximal femoral component 10 and thepatient's femur.

Substrate 20 of proximal femoral component 10 may comprise abiocompatible metal, such as titanium, a titanium alloy, cobaltchromium, cobalt chromium molybdenum, tantalum, or a tantalum alloy.According to an exemplary embodiment of the present disclosure,substrate 20 comprises a Ti-6Al-4V ELI alloy, such as Tivanium® which isavailable from Zimmer, Inc., of Warsaw, Ind. Tivanium® is a registeredtrademark of Zimmer, Inc.

Porous layer 22 of proximal femoral component 10 may comprise abiocompatible metal, such as titanium, a titanium alloy, cobaltchromium, cobalt chromium molybdenum, tantalum, or a tantalum alloy.Porous layer 22 may be in the form of a highly porous biomaterial, whichis useful as a bone substitute and as cell and tissue receptivematerial. It is also within the scope of the present disclosure thatporous layer 22 may be in the form of a fiber metal pad or a sinteredmetal layer, such as a Cancellous-Structured Titanium™ (CSTi™) layer,for example. CSTi™ porous layers are manufactured by Zimmer, Inc., ofWarsaw, Ind. Cancellous-Structured Titanium™ and CSTi™ are trademarks ofZimmer, Inc.

A highly porous biomaterial may have a porosity as low as 55%, 65%, or75% and as high as 80%, 85%, or 90%, or within any range defined betweenany pair of the foregoing values. An example of such a material is ahighly porous fiber metal pad. Another example of such a material is aCSTi™ layer. Yet another example of such a material is produced usingTrabecular Metal™ technology generally available from Zimmer, Inc., ofWarsaw, Ind. Trabecular Metal™ is a trademark of Zimmer, Inc. Such amaterial may be formed from a reticulated vitreous carbon foam substratewhich is infiltrated and coated with a biocompatible metal, such astantalum, by a chemical vapor deposition (“CVD”) process in the mannerdisclosed in detail in U.S. Pat. No. 5,282,861, the disclosure of whichis expressly incorporated herein by reference. In addition to tantalum,other metals such as niobium, or alloys of tantalum and niobium with oneanother or with other metals may also be used.

Generally, the porous tantalum structure includes a large plurality ofligaments defining open spaces therebetween, with each ligamentgenerally including a carbon core covered by a thin film of metal suchas tantalum, for example. The open spaces between the ligaments form amatrix of continuous channels having no dead ends, such that growth ofcancellous bone through the porous tantalum structure is uninhibited.The porous tantalum may include up to 75%-85% or more void spacetherein. Thus, porous tantalum is a lightweight, strong porous structurewhich is substantially uniform and consistent in composition, andclosely resembles the structure of natural cancellous bone, therebyproviding a matrix into which cancellous bone may grow to providefixation of proximal femoral component 10 to the patient's femur.

The porous tantalum structure may be made in a variety of densities inorder to selectively tailor the structure for particular applications.In particular, as discussed in the above-incorporated U.S. Pat. No.5,282,861, the porous tantalum may be fabricated to virtually anydesired porosity and pore size, and can thus be matched with thesurrounding natural bone in order to provide an optimized matrix forbone ingrowth and mineralization.

When porous layer 22 of proximal femoral component 10 is produced usingTrabecular Metal™ technology, as discussed above, a small percentage ofsubstrate 20 may be in direct contact with the ligaments of porous layer22. For example, approximately 15%, 20%, or 25%, of the surface area ofsubstrate 20 may be in direct contact with the ligaments of porous layer22.

Referring next to FIG. 3, apparatus 100 is provided for resistancewelding porous layer 22 to substrate 20 of proximal femoral component10. Apparatus 100 is also illustrated schematically in FIGS. 4A and 4B.Apparatus 100 includes housing 110, one or more braces or fixtures 120a, 120 b, one or more weld heads 130 a, 130 b, within housing 110, eachhaving an electrode 132 a, 132 b, transformer 140, a power source orcurrent generator 150, and controller 160. Each component of apparatus100 is described further below.

Housing 110 of apparatus 100 defines an internal chamber 112 that issized to receive at least one prosthesis, such as proximal femoralcomponent 10 of FIGS. 1 and 2. According to an exemplary embodiment ofthe present disclosure, housing 110 of apparatus 100 creates a vacuumenvironment or an inert environment in chamber 112 during the resistancewelding process. In one particular example, chamber 112 of housing 110is flushed with an inert gas (e.g., argon) and controlled to have a dewpoint less than about −60° C. and an oxygen concentration less thanabout 10 ppm.

Housing 110 may be at least partially transparent to enable a user tosee inside chamber 112. Also, housing 110 may include one or moreopenings 114 to enable a user to access chamber 112. To maintain avacuum environment or an inert environment in chamber 112, housing 110may be in the form of a glovebox. In other words, each opening 114 mayinclude a glove (not shown) or another suitable barrier that extendsinto chamber 112 to receive the user's hand while maintaining a sealaround opening 114.

Fixtures 120 a, 120 b, of apparatus 100 contact proximal femoralcomponent 10 to hold proximal femoral component 10 in place withinhousing 110 of apparatus 100. Fixtures 120 a, 120 b, may be moved apartto an open position (FIG. 4A) to receive proximal femoral component 10,and then fixtures 120 a, 120 b, may be moved together to a closed orclamped position (FIG. 4B) to hold proximal femoral component 10 inplace. It is within the scope of the present disclosure that the closedposition of fixtures 120 a, 120 b, may be adjustable to enable apparatus100 to receive and hold prostheses of different shapes and sizes.

Electrodes 132 a, 132 b, on weld heads 130 a, 130 b, of apparatus 100are connected to transformer 140 and current generator 150 via wires 152a, 152 b, respectively. As shown in FIG. 4A, each electrode 132 a, 132b, faces a corresponding side of porous layer 22. More specifically,contact surface 134 a, 134 b, of each electrode 132 a, 132 b, faces acorresponding side of porous layer 22. According to an exemplaryembodiment of the present disclosure, contact surface 134 a, 134 b, ofeach electrode 132 a, 132 b, is designed to substantially match thecontour of the corresponding side of porous layer 22. In thisembodiment, each electrode 132 a, 132 b, is able to make close, evencontact with proximal femoral component 10. Depending on the shape ofproximal femoral component 10, the corresponding contact surface 134 a,134 b, may be concave, convex, or planar, for example.

In some embodiments, an electrode contact surface such as contactsurfaces 134 a, 134 b will be particularly textured, e.g., incorporateone or more surface features or elements, or will otherwise beconfigured so that structural and/or other characteristics (e.g.,surface texture, strut features, surface porosity, pore features, etc.)of a porous metal structure contacted by the electrode during a weldingstep will be preserved or substantially preserved, when suchpreservation is desired, after the welding step has been completed.Illustratively, an electrode surface can be equipped with a particularmicroarchitecture that helps prevent or inhibit structural changes fromoccurring on or within a porous metal structure during a welding step.With some designs, employing this sort of surface, as compared to usinga smooth, even, or generally non-textured surface of the same generalsize, can reduce the amount of contact between the electrode and theporous metal, and in this regard, it will be understood that the amountof contact between an electrode and a porous metal over a given area canbe set at any suitable level. Such texturing, etc. can be incorporatedinto an electrode surface upon initial formation of the electrode, or anexisting electrode surface can be modified to have such features. Inthis regard, such texturing, etc. can be imparted to an electrodesurface, for example, by cutting away, grinding away or otherwiseremoving material from an initial electrode piece to provide aparticular surface texture or other surface microarchitecture, or bywelding, adhering or otherwise adding material to an existing electrodepiece to provide a particular surface texture or other surfacemicroarchitecture, or by casting or otherwise initially forming anelectrode piece to have a particular surface texture or other surfacemicroarchitecture. In some forms, a knurled electrode surface will beutilized. Such texturing, etc. can include any number and type ofsurface features or elements including but not limited to one or moreprojections, grooves, ridges, corrugations, peaks, valleys, rings,bands, bumps, bulges, lumps, knobs, protuberances, dimples, depressions,dents, and/or other suitable projections or indentations. Such surfacefeatures or elements can be of any suitable size and shape, and they canoccur randomly along an electrode surface, or they can be arranged inregular or non-random fashion (e.g., a pattern, grid or array) along anelectrode surface or region. In some forms, an electrode surface will betextured or otherwise configured so that it somewhat mimics orapproximates one or more surface features of a porous metal structure tobe contacted by the electrode during a welding step.

Referring now to FIGS. 16-18, the first of these figures is amicroscopic image of a control sample of a porous metal, while FIG. 17is a microscopic image of the same type and grade of porous metal as inFIG. 16 except that the porous metal was contacted by an electrodehaving a textured surface (45 degree, 0.040 pitch) during a welding stepas described herein. A comparison of FIGS. 16 and 17 shows a substantialpreservation of features of the porous metal structure. FIG. 18 is amicroscopic image of the same type and grade of porous metal as in FIG.16 except that the porous metal was contacted by an electrode having anon-textured surface during a welding step as described herein. Acomparison of FIGS. 17 and 18 shows a considerable difference inpreservation of features of the porous metal structure.

With reference now to FIG. 19, shown is an illustrative electrode 200according to one aspect of the present disclosure. Electrode 200includes an electrode contact surface 201 from which a plurality ofgenerally pyramid-shaped projections project. These particularpyramid-like projections 202, while certainly useful in certainembodiments of the present disclosure, are merely illustrative of thetype, shape, etc. of projections contemplated. Projections and othersuitable surface elements or features of any suitable shape, size andconfiguration can be incorporated into an electrode surface as discussedelsewhere herein.

Weld heads 130 a, 130 b, of apparatus 100 may be configured to holdporous layer 22 against substrate 20 during the resistance weldingprocess. More particularly, weld heads 130 a, 130 b, may be configuredto hold porous layer 22 within recess 26 of substrate 20 during theresistance welding process. Like fixtures 120 a, 120 b, described above,weld heads 130 a, 130 b, may be moved away from proximal femoralcomponent 10 to an open position (FIG. 4A) to receive proximal femoralcomponent 10, and thenweld heads 130 a, 130 b, may be moved towardproximal femoral component 10 to a closed or clamped position (FIG. 4B)to hold porous layer 22 within recess 26 of substrate 20. The openand/or closed positions of weld heads 130 a, 130 b, may be controlledusing one or more stops 136 a, 136 b, that contact corresponding flanges138 a, 138 b, on weld heads 130 a, 130 b, to limit movement ofelectrodes 132 a, 132 b. It is within the scope of the presentdisclosure that the closed position of each weld head 130 a, 130 b, maybe adjustable, such as by moving stops 136 a, 136 b, to enable apparatus100 to receive and hold prostheses of different shapes and sizes.

Optionally, apparatus 100 may include additional braces or fixtures (notshown) that are configured to hold porous layer 22 against substrate 20during the resistance welding process. More particularly, theseadditional fixtures may be configured to hold porous layer 22 withinrecess 26 of substrate 20 during the resistance welding process.

The pressure used to hold porous layer 22 against substrate 20 ofproximal femoral component 10 during the resistance welding process maybe sufficiently low to avoid deforming or compressing porous layer 22while still resisting movement of porous layer 22 relative to substrate20. Thus, the weld pressure should not exceed the compressive yieldstrength of substrate 20 or porous layer 22. If the compressive yieldstrength of porous layer 22 is about 4,000 psi (27.6 MPa), for example,a suitable weld pressure may be as low as 100 psi (0.7 MPa), 500 psi(3.4 MPa), or 1,000 psi (6.9 MPa), and as high as 2,000 psi (13.8 MPa),2,500 psi (17.2 MPa), or 3,000 psi (20.7 MPa), or within any rangedefined between any pair of the foregoing values, for example. Porouslayer 22 may be provided in a substantially final shape before theresistance welding process to avoid having to compress or otherwiseshape porous layer 22 during the resistance welding process. As aresult, the thickness of porous layer 22 and the contact area betweenporous layer 22 and substrate 20 may remain substantially unchangedduring the resistance welding process. As discussed above, the weldpressure may be applied by weld heads 130 a, 130 b, when weld heads 130a, 130 b, are in the closed position (FIG. 4B) and/or by additionalfixtures (not shown) of apparatus 100.

Controller 160 of apparatus 100, which may be in the form of a generalpurpose computer, is coupled to transformer 140 and current generator150 to control the operation of electrodes 132 a, 132 b. Controller 160of apparatus 100 may also control the evacuation of housing 110 and/orthe flushing of housing 110 with an inert gas (e.g., argon).Additionally, controller 160 of apparatus 100 may control movement offixtures 120 a, 120 b, and/or weld heads 130 a, 130 b, between theirrespective open positions (FIG. 4A) and closed positions (FIG. 4B).

In use, proximal femoral component 10 is loaded into housing 110 ofapparatus 100. With porous layer 22 properly disposed against substrate20 of proximal femoral component 10, controller 160 may be operated tomove fixtures 120 a, 120 b, and/or weld heads 130 a, 130 b, from theirrespective open positions (FIG. 4A) toward their respective closedpositions (FIG. 4B). The approach pressure (i.e. the pressure at whichfixtures 120 a, 120 b, and/or weld heads 130 a, 130 b, approach proximalfemoral component 10 before making contact with proximal femoralcomponent 10) may be less than the above-described weld pressure toavoid damaging the components. For example, the approach pressure may beas low as 10 psi (0.07 MPa), 30 psi (0.2 MPa), or 50 psi (0.3 MPa), andas high as 70 psi (0.5 MPa), 90 psi (0.6 MPa), or 110 psi (0.8 MPa), orwithin any range defined between any pair of the foregoing values.

After proximal femoral component 10 is loaded into housing 110 ofapparatus 100, controller 160 may be operated to evacuate chamber 112 ofhousing 110 and/or to flush chamber 112 of housing 110 with an inert gas(e.g., argon). The vacuum or inert environment within housing 110 ofapparatus 100 may substantially prevent proximal femoral component 10from oxidizing, absorbing atmospheric contaminants, and/or becomingdiscolored during the resistance welding process.

Controller 160 may then continue moving fixtures 120 a, 120 b, and weldheads 130 a, 130 b, into their respective closed positions (FIG. 4B) tohold both porous layer 22 and substrate 20 of proximal femoral component10 in place within housing 110. As discussed above, the weld pressure(i.e., the pressure at which fixtures 120 a, 120 b, and/or weld heads130 a, 130 b, come to hold proximal femoral component 10 during theresistance welding process) may be as low as 100 psi (0.7 MPa), 500 psi(3.4 MPa), or 1,000 psi (6.9 MPa), and as high as 2,000 psi (13.8 MPa),2,500 psi (17.2 MPa), or 3,000 psi (20.7 MPa), for example.

Next, controller 160 may be operated to initiate current flow fromcurrent generator 150 to transformer 140. Current generator 150 mayoperate at a power of 4 kJ, 6 kJ, 8 kJ, 10 kJ, or more, for example.With a textured or non-textured contact surface 134 a, 134 b, of eachelectrode 132 a, 132 b, positioned against porous layer 22 of proximalfemoral component 10, the weld current flows from one electrode (e.g.,electrode 132 a via wire 152 a), through proximal femoral component 10,and out of the other electrode (e.g., electrode 132 b via wire 152 b).In an exemplary embodiment, the source electrode 132 a, 132 b, maydeliver a weld current to proximal femoral component 10 as low as 20 kA,30 kA, or 40 kA, and as high as 50 kA, 60 kA, or 70 kA, or within anyrange defined between any pair of the foregoing values, for example, toproduce weld current densities as low as 25 kA/in² (3.9 kA/cm²), 35kA/in² (5.4 kA/cm²), or 45 kA/in² (7.0 kA/cm²), and as high as 55 kA/in²(8.5 kA/cm²), 65 kA/in² (10.1 kA/cm²), 75 kA/in² (11.6 kA/cm²), or 85kA/in² (13.2 kA/cm²), or within any range defined between any pair ofthe foregoing values. As the weld current flows through proximal femoralcomponent 10, controller 160 may maintain the weld pressure of fixtures120 a, 120 b, and/or weld heads 130 a, 130 b.

According to Ohm's Law (P=I²*R), the weld current (I) that flows throughporous layer 22 and substrate 20 of proximal femoral component 10dissipates as heat, with the amount of heat generated being proportionalto the resistance (R) at any point in the electrical circuit. Whendifferent materials are used to construct porous layer 22 and substrate20, the resistance (R) may be highest at the interface between porouslayer 22 and substrate 20. Therefore, a large amount of heat may begenerated locally at points of contact between porous layer 22 andsubstrate 20.

According to an exemplary embodiment of the present disclosure, the heatgenerated is sufficient to cause softening and/or melting of thematerials used to construct porous layer 22 and/or substrate 20 which,in combination with the weld pressure used to hold porous layer 22against substrate 20, causes surface metallurgical bonding to occur atpoints of contact between porous layer 22 and substrate 20. It is alsowithin the scope of the present disclosure that metallurgical bondingmay occur at points of contact within porous layer 22. For example, ifporous layer 22 is in the form of a fiber metal pad, metallurgicalbonding may occur at points of contact between adjacent metal wireswithin the fiber metal pad.

According to another exemplary embodiment of the present disclosure, theweld current may be delivered to proximal femoral component 10 indiscrete but rapid pulses. The weld current may be delivered to proximalfemoral component 10 with as few as 4, 6, or 8 pulses and as many as 10,12, or 14 pulses, or any value therebetween, for example. Each pulse maybe as short as 20 milliseconds, 40 milliseconds, or 60 milliseconds, andas long as 80 milliseconds, 100 milliseconds, or 120 milliseconds, orany value therebetween, for example. Between each pulse, the absence ofa weld current may promote bulk cooling of porous layer 22 and substrate20 without eliminating localized, interfacial heating of porous layer 22and substrate 20. The cooling time between each pulse may be less than 1second, and more specifically may be as short as 20 milliseconds, 40milliseconds, or 60 milliseconds, and as long as 80 milliseconds, 100milliseconds, or 120 milliseconds, or any value therebetween, forexample.

As discussed above, the weld pressure used to hold porous layer 22against substrate 20 during the resistance welding process should besufficiently low to avoid deforming porous layer 22. Due to softeningand/or melting of substrate 20 along the interface, porous layer 22 mayshift or translate slightly toward the softened substrate 20 and maybecome embedded within the softened substrate 20. Therefore, the totalthickness of proximal femoral component 10 (i.e., the combined thicknessof porous layer 22 and substrate 20) may decrease during the resistancewelding process. For example, the total thickness of proximal femoralcomponent 10 may decrease by approximately 0.1%, 0.2%, 0.3%, or more,during the resistance welding process. However, the thickness of porouslayer 22 itself should not substantially change. In other words, anymeasurable change in thickness of proximal femoral component 10 shouldresult from porous layer 22 shifting into the softened substrate 20, notfrom the compaction or deformation of porous layer 22 itself. Whenporous layer 22 is in the form of a fiber metal pad, porous layer 22 mayundergo some deformation (e.g., shrinkage) due to the formation ofmetallurgical bonds within porous layer 22. However, this deformationshould not be attributed to the weld pressure.

After delivering current to proximal femoral component 10, substrate 20and porous layer 22 will begin to cool. During this time, controller 160may be operated to maintain a forge pressure on the components. Theforge pressure (i.e. the pressure at which fixtures 120 a, 120 b, and/orweld heads 130 a, 130 b, hold proximal femoral component 10 after theweld current ceases) may be less than the above-described weld pressure.For example, the forge pressure may be as low as 40 psi (0.3 MPa), 60psi (0.4 MPa), or 80 psi (0.6 MPa) and as high as 100 psi (0.7 MPa), 120psi (0.8 MPa), or 140 psi (1.0 MPa), or within any range defined betweenany pair of the foregoing values. The forge time may be as short as 1second, 2 seconds, or 3 seconds, and as long as 4 seconds, 5 seconds, ormore, for example.

In total, the time required to resistance weld porous layer 22 tosubstrate 20 using apparatus 100 may be as short as 1 second, 10seconds, 20 seconds, or 30 seconds, and as long as 1 minute, 2 minutes,3 minutes, or more, for example. The time required may vary depending onthe thickness of porous layer 22, the current generated by currentgenerator 150, and other parameters.

Finally, controller 160 may be operated to return fixtures 120 a, 120 b,and/or weld heads 130 a, 130 b, to their respective open positions (FIG.4A). Proximal femoral component 10 may then be removed from housing 110of apparatus 100 with porous layer 22 fixedly secured to substrate 20.

Advantageously, by resistance welding porous layer 22 onto substrate 20,a strong metallurgical bond may be achieved between porous layer 22 andsubstrate 20. In certain embodiments, the bond strength between porouslayer 22 and substrate 20 may be at least 2,900 psi (20.0 MPa), which isthe FDA-recommended bond strength for orthopedic implants. Also, becauseresistance welding involves localized, interfacial heating of porouslayer 22 and substrate 20 and requires short cycle times, degradation ofporous layer 22 and substrate 20 may be avoided. As a result, thefatigue strength of substrate 20 and porous layer 22 may besubstantially unchanged during the resistance welding process.

Although porous layer 22 is described and depicted herein as beingbonded directly to substrate 20 of proximal femoral component 10, it isalso within the scope of the present disclosure that porous layer 22 maybe pre-bonded to an intermediate layer (not shown), which is thensubsequently bonded to substrate 20. A suitable intermediate layer mayinclude, for example, a titanium foil. Both the pre-bonding step betweenporous layer 22 and the intermediate layer and the subsequent bondingstep between the intermediate layer and substrate 20 may involveresistance welding, as described above with reference to FIGS. 3, 4A,and 4B. However, it is also within the scope of the present disclosurethat the subsequent bonding step between the intermediate layer andsubstrate 20 may involve traditional, diffusion bonding.

EXAMPLES 1. Example #1 Analysis of Trabecular Metal™ Surface Finish andThickness

A series of samples were prepared, each sample having a disc-shapedporous component produced using Trabecular Metal™ technology and adisc-shaped Tivanium® substrate. The substrates were substantially thesame, but the porous components differed in two aspects—surface finishand thickness—as set forth in Table 1 below.

TABLE 1 Interfacing Surface Finish of Porous Component Group PorousComponent Thickness (inches) 1 A Electro discharge machined (EDM) 0.060B Electro discharge machined (EDM) 0.125 C Electro discharge machined(EDM) 0.250 2 A Net shape 0.060 B Net shape 0.125 C Net shape 0.250 3 ASmeared 0.060 B Smeared 0.125 C Smeared 0.250

Before placing the interfacing surface of each porous component againstits corresponding substrate, the interfacing surface of each porouscomponent was treated as set forth in Table 1 above.

In Group 1, the interfacing surface of each porous component wassubjected to electro discharge machining (EDM), which broke off someprotruding ligaments of the porous component and leveled the interfacingsurface, making more ligaments available at the interfacing surface tocontact the underlying substrate. Therefore, EDM moderately increasedthe net contact area of the porous components in Group 1.

In Group 2, each porous component was provided in a net shape and theinterfacing surface of each porous component was not subjected tomachining after manufacturing, so the bulk porosity of the porouscomponent was retained at the interfacing surface. More specifically,the net-shaped interfacing surface was produced by coating an outersurface of a porous structure (e.g., a reticulated vitreous carbon foamstructure) with metal and then maintaining the outer, coated surfacewithout machining or shaping the outer surface. Therefore, the netcontact area of the porous components in Group 2 was retained.

In Group 3, the interfacing surface of each porous component wassubjected to physical machining to break off some ligaments of theporous component and spread out or “smear” other ligaments of the porouscomponent, which caused a substantial reduction in the surface porosityof the interfacing surface. Therefore, smearing increased the netcontact area of the porous components in Group 3.

As a result of these surface treatments, the porous components of Group2 had the least surface contact with the underlying substrate, while theporous components of Group 3 had the most surface contact with theunderlying substrate.

The samples were then assembled by resistance welding. A first quantityof power was applied to weld the 0.060 inch (1.5 mm) thick and the 0.125inch (3.2 mm) thick porous components (Groups 1A, 1B, 2A, 2B, 3A, and3B) to their corresponding substrates. A second quantity of power 50%greater than the first quantity of power was applied to weld the 0.250inch (6.4 mm) thick porous components (Groups 1C, 2C, and 3C) to theircorresponding substrates. The average bond strength for the samples ofeach Group 1-3 is depicted graphically in FIG. 5.

As shown in FIG. 5, the samples of Group 2 had higher average bondstrengths than the samples of Groups 1 or 3. Because the porouscomponents of Groups 1 and 3 had more surface contact with theunderlying substrates than the samples of Group 2, the inventors suspectthat the applied current and heat dissipated across the greater surfacecontact area, resulting in weaker bonds for the samples of Groups 1 and3 than the samples of Group 2. In contrast, because the porouscomponents of Group 2 had less surface contact with the underlyingsubstrates than the samples of Groups 1 and 3, the inventors suspectthat the applied current and heat was localized at each individualligament, resulting in stronger bonds for the samples of Group 2 thanthe samples of Groups 1 and 3.

Also, within each Group 1-3, the samples of subgroups A and C had higheraverage bond strengths than the samples of the corresponding subgroup B.For example, the samples of Groups 2A and 2C had higher average bondstrengths than the samples of Group 2B.

The decrease in bond strength from subgroup A to B within each Group 1-3may be attributed to the increased thickness of the porous componentsfrom 0.060 inch to 0.125 inch. Because the thermal conductivity oftantalum in each porous component (about 54 W/m/K) is greater than thethermal conductivity of titanium in each substrate (about 7 W/m/K), thethicker porous components of each subgroup B may act as heat sinks,conducting the heat generated at the interface away from the interfaceand into the volume of the porous component.

The increase in bond strength from subgroup B to C within each Group 1-3may be attributed to the 50% increase between the first quantity ofpower used to resistance weld the 0.060 inch thick and the 0.125 inchthick porous components and the second quantity of power used toresistance weld the 0.250 inch thick porous components. The increasedpower produces increased current flow, which results in greater heatingand a stronger bond.

2. Example #2 Analysis of Trabecular Metal™ Thickness, Weld Power, andNumber of Weld Cycles

Another series of samples were prepared, each sample having adisc-shaped porous component produced using Trabecular Metal™ technologyand a disc-shaped Tivanium® substrate. Because the net-shaped porouscomponents (Group 2) achieved the highest bond strengths in Example #1,the porous components of Example #2 were also provided in a net shape.The substrates were substantially the same, but the porous componentsdiffered in thickness. Also, the resistance welding process differed intwo aspects—power and number of weld cycles—as set forth in Table 2below.

TABLE 2 Porous Component Weld Power Number of Group Thickness (inches)(kJ) Weld Cycles 4 A 0.060 6 1 B 0.060 6 2 5 A 0.125 6 1 B 0.125 6 2 6 A0.125 9 1 B 0.125 9 2

The average bond strength for the samples of each Group 4-6 is depictedgraphically in FIG. 6. The samples of Groups 4 and 6 had higher averagebond strengths than the samples of Group 5. In fact, the samples ofGroups 4 and 6 had average bond strengths above 4,000 psi (27.6 MPa),which exceeds the FDA-recommended bond strength of 2,900 psi (20.0 MPa).

About 90.6% of the variation in bond strength may be attributed to thevaried thickness of the porous components and the varied weld power. Thenumber of weld cycles was found to be statistically insignificant.

3. Example #3 Analysis of Trabecular Metal™ Thickness and Weld Time

Another series of circular samples were prepared, each sample having adisc-shaped porous component produced using Trabecular Metal™ technologyand a disc-shaped Tivanium® substrate. The contact area between eachporous component and its underlying substrate was about 5 square inches(32.3 cm²). The substrates were substantially the same, but the porouscomponents differed in thickness. Also, the resistance welding cycletime differed, as set forth in Table 3 below.

TABLE 3 Porous Component Weld Time Group Thickness (inches)(milliseconds) 7 A 0.060 150 B 0.060 200 C 0.060 250 8 A 0.125 150 B0.125 200 C 0.125 250

After resistance welding the samples, two 1.2 inch (3.0 cm) diametertest coupons were cut from each sample for tensile testing. The bondstrength for each test coupon of Groups 7 and 8 is depicted graphicallyin FIG. 7. As shown in FIG. 7, one of the two test coupons of Group 8Bhad a bond strength greater than the FDA-recommended bond strength of2,900 psi (20.0 MPa). However, the other test coupon of Group 8B had abond strength less than 1,000 psi (6.9 MPa).

The variation in bond strength between corresponding test coupons may bedue to non-uniform pressure and/or current across each sample. Physicalexamination of the remnant material left behind after cutting the 1.2inch (3.0 cm) diameter test coupons supported the finding of varyinglevels of bond strength within each sample.

4. Example #4 Comparison Between Resistance Welding and DiffusionBonding

In addition to tensile testing, metallography was also performed tocompare the bond achieved with resistance welding to the bond achievedwith diffusion bonding.

When a porous component is diffusion bonded to an underlying substrate,atoms from the porous component and atoms from the substrateinter-diffuse. For example, when a porous component produced usingTrabecular Metal™ technology is diffusion bonded to a Tivanium®substrate, tantalum from the porous component diffuses into thesubstrate, and titanium from the substrate diffuses into the porouscomponent. The diffusion of tantalum into the substrate is showngraphically in FIG. 8, and the diffusion of titanium into the porouscomponent is shown graphically in FIG. 9. The inter-diffusion oftantalum and titanium creates a concentration gradient or aninter-diffusion layer along the interface between the porous componentand the substrate. The inter-diffusion layer between the porouscomponent and the substrate is also shown visually in FIG. 10, which isa scanning electron microscope image taken along the interface betweenthe porous component and the substrate of a diffusion bonded sample.

When a porous component is resistance welded to an underlying substrate,little or no inter-diffusion occurs. For example, the tantalumconcentration in the porous component remains substantially constant inFIG. 8, and the titanium concentration in the substrate remainssubstantially constant in FIG. 9. The lack of any significantinter-diffusion layer between the porous component and the substrate isalso shown visually in FIG. 11, which is a scanning electron microscopeimage taken along the interface between the porous component and thesubstrate of a resistance welded sample.

5. Example #5 Analysis of Weld Pressure

A series of 1 inch (2.5 cm) diameter, disc-shaped samples were prepared,the electrode interface of each sample having a surface area of about0.79 square inches (5.1 cm²). Each sample had a 0.055 inch (1.4 mm)thick porous component produced using Trabecular Metal™ technology and a0.130 inch (3.3 mm) thick Tivanium® substrate. The weld pressure wascalculated to be 4,160 psi (28.7 MPa), which was comparable to thecompressive yield strength of the porous components. As a result of thishigh weld pressure, the porous components were partially crushed duringwelding and decreased in average thickness by about 0.022 inch (0.6 mm)or 40% (from 0.055 inch (1.4 mm) to 0.033 inch (0.8 mm)).

6. Example #6 Analysis of Pulse Welding

Another series of 1 inch (2.5 cm) diameter, disc-shaped samples wereprepared, each sample having a 0.055 inch (1.4 mm) thick porouscomponent produced using Trabecular Metal™ technology and a 0.130 inch(3.3 mm) thick Tivanium® substrate. Each porous component included anEDM-shaped surface interfacing with the substrate. The resistancewelding parameters of Example #6 are set forth in Table 4 below.

TABLE 4 Weld Parameter Setting Approach Pressure 20 psi Approach Time 3seconds Weld Pressure 800 psi Forge Pressure 45 psi Forge Time 3 secondsCurrent 24 kA Current Density 30 kA/in² Controlled Atmosphere argon dewpoint <−60° C. oxygen concentration <10 ppm

As set forth in Table 5 below, each sample received a different numberof weld current pulses, with each pulse lasting 80 milliseconds and thecooling time between each pulse lasting 80 milliseconds. Samples 1-5were prepared to evaluate up to 10 pulses. Samples 6-11 were prepared tomore specifically evaluate between 5 and 10 pulses.

TABLE 5 Sample Number of Pulses 1 1 2 4 3 6 4 8 5 10 6 5 7 6 8 7 9 8 109 11 10

A new resistance welding apparatus was designed and built to deliverthese weld current pulses in a controlled environment. The apparatusincluded the AX5000 Atmospheric Enclosure with the BMI-500 Single-ColumnGas Purification System, the KN-II Projection Weld Head with cooled,copper alloy electrodes, the IT-1400-3 Transformer, and the ISA-2000CRInverter Power Supply, all of which are available from Miyachi UnitekCorporation of Monrovia, Calif.

The overall thickness of each sample remained substantially the samebefore and after welding, indicating that the lower 800 psi (5.5 MPa)weld pressure of Example #5 successfully eliminated the distortion andcrushing of the porous component seen in Example #4 above.

Samples 1-5 were subjected to tensile testing. The tensile strength ofthe weld increased from 0 psi (0 MPa) for Sample 1 (1 pulse) to 6,882psi (47.4 MPa) for Sample 3 (6 pulses). The tensile strength of the weldremained approximately the same for Sample 4 (8 pulses) and Sample 5 (10pulses).

Samples 6-11 were then subjected to tensile testing and regressionanalysis, the results of which are presented graphically in FIG. 12. Asshown in FIG. 12, the bond strength increased with each additionalpulse. The lower 95% prediction interval intersects the 2,900 psi (20.0MPa) reference line above 9 weld pulses (see circled intersection pointin FIG. 12). Thus, at the given weld parameters, at least 10 weld pulseswould be required to consistently produce bond strengths of at least2,900 psi.

Visual inspection of the samples revealed the formation of noticeableheat-affected zones along both the bond interface (i.e., the interfacebetween the porous component and the substrate) and the electrodeinterface (i.e., the interface between the sample and the resistancewelding electrode). Due to the heat generated during the resistancewelding process, the samples may experience discoloration,bleed-through, and/or microstructure changes in such heat-affectedzones. FIG. 13, for example, depicts the heat-affected zone that formedalong the bond interface of Sample 1 above. It would be possible andwithin the scope of the present disclosure to machine away or otherwiseremove the electrode interface after the resistance welding process.However, it would not be possible to remove the bond interface after theresistance welding process without destroying the bond.

7. Example #7 Analysis of Pulsed Weld Current to Reduce Heat-AffectedZones for Net-Shaped Porous Components

To eliminate the heat-affected zones seen in Example #6 above, anotherseries of 1 inch (2.5 cm) diameter, disc-shaped samples were prepared,each sample having a 0.055 inch (1.4 mm) thick porous component producedusing Trabecular Metal™ technology and a 0.130 inch (3.3 mm) thickTivanium® substrate. Unlike Example #6, each porous component included anet-shaped, not EDM-shaped, interfacing with the substrate. Also,compared to Example #6, the samples were subjected to shorter weldpulses, longer cooling times between pulses, higher weld pressures, andhigher weld currents during resistance welding. The resistance weldingparameters of Example #7 are set forth in Table 6 below.

TABLE 6 Weld Parameter Setting Approach Pressure 20 psi Approach Time 3seconds Weld Pressure 1,000 psi Forge Pressure 62 psi Forge Time 3seconds Number of Pulses 10 Weld Time per Pulse 15 milliseconds CoolingTime 250 milliseconds Controlled Atmosphere argon dew point <−60° C.oxygen concentration <10 ppm

As set forth in Table 7 below, the samples received pulsed weld currentsbetween 35 kA and 51 kA.

TABLE 7 Actual Weld Current Weld Current (kA) Sample Setting (kA) (forPulse 1) (for Pulse 2-10) 1a 35 33.9 34.3 1b 35 33.9 34.3 2a 39 37.538.0 2b 39 37.6 38.0 3a 43 41.0 41.7 3b 43 41.2 41.6 4a 47 43.7 45.1 4b47 44.3 45.2 5a 51 46.9 48.5 5b 51 46.9 48.5

As an initial matter, visual inspection of the samples showed that thedepth and extent of the heat-affected zones along the bond interfaceswere significantly reduced compared to Example #6 or, in some cases,were eliminated altogether. For example, Sample 1 of Example #6 (FIG.13) has a noticeably larger heat-affected zone than Sample 3b of Example#7 (FIG. 14). Some heat-affected zones remained along the electrodeinterfaces, but as discussed above, it would be possible to machine awayor otherwise remove these electrode interfaces after the resistancewelding process.

The samples were also subjected to tensile testing and regressionanalysis, the results of which are presented graphically in FIG. 15. Asshown in FIG. 15, the bond strength increased as the weld current perpulse increased. The lower 95% prediction interval intersects the 2,900psi (20.0 MPa) reference line at about 43 kA per pulse (see circledintersection point in FIG. 13). Thus, at the given weld parameters, aweld current of at least 43 kA per pulse (or a weld density of at least54 kA/in² (8.4 kA/cm²) per pulse) would be required to consistentlyproduce bond strengths of at least 2,900 psi.

Additional samples were welded at 46 kA per pulse (or a weld density ofabout 58 kA/in² (9.0 kA/cm²) per pulse) to confirm this result, but thebond strengths were inconsistent and ranged from 2,387 psi (16.5 MPa) to4,246 psi (29.3 MPa). Also, visual inspection of these additionalsamples revealed noticeable heat-affected zones along the bondinterfaces. The inventors attribute these inconsistent results, at leastpartially, to electrode wear and metal transfer onto the electrode.

8. Example #8 Analysis of Pulsed Weld Current to Reduce Heat-AffectedZones for EDM-Shaped Porous Components

Example #7 was repeated with porous components having EDM-shaped (notnet-shaped) surfaces interfacing with the substrate. None of the samplesreached a bond strength of 2,900 psi (20.0 MPa). Also, the samplesformed noticeable heat-affected zones along the bond interfaces.

9. Example #9 Analysis of Weld Pressure and Pulsed Weld Current toReduce Electrode Damage and Improve Bond Strength

To improve the results of Example #7, including reducing generalelectrode wear and metal transfer onto the electrode, Example #7 wasrepeated at a higher approach pressure of 40 psi (0.3 MPa), a higherweld pressure of 2,000 psi (13.8 MPa), and a lower forge pressure of 55psi (0.4 MPa). In accordance with Example #7, the samples were subjectedto pulsed weld currents greater than 43 kA, specifically between 45 kAand 61 kA.

At the higher weld pressure of Example #9 (2,000 psi), the inventorsnoticed less sticking between the samples and the electrodes than at thelower weld pressure of Example #7 (1,000 psi). The present inventorsbelieve that the higher weld pressures increased contact between thesamples and the electrodes, and therefore reduced the resistance and theheat generated between the samples and the electrodes.

The samples were subjected to tensile testing and regression analysis,which indicated that a weld current of at least 59 kA per pulse (or aweld density of at least 75 kA/in² (11.6 kA/cm²) per pulse) would berequired to consistently produce bond strengths of at least 2,900 psi(20.0 MPa).

Nine additional samples were welded at about 59 kA per pulse to confirmthis result, and the bond strengths of these nine additional samplesaveraged 4,932 psi (34.0 MPa), ranging from 3,174 psi (21.9 MPa) to6,688 psi (46.1 MPa). Also, visual inspection of these nine additionalsamples showed minimal presence of heat-affected zones along the bondinterfaces.

Six additional samples were welded at about 61 kA per pulse (or a welddensity of about 77 kA/in² (11.9 kA/cm²) per pulse) to further confirmthis result. Each of these additional samples had a slighter thickersubstrate, specifically a 0.170 inch (4.3 mm) thick substrate, comparedto previous tests, where the substrates were 0.130 inch thick (3.3 mm).The bond strengths of these additional samples averaged 3,968 psi (27.4MPa), ranging from 3,259 psi (22.5 MPa) to 4,503 psi (31.0 MPa). In allof these additional samples, tensile failure occurred within the porousmaterial, not along the bond interface between the porous material andthe substrate. Also, visual inspection of these samples showed nomacroscopic heat-affected zones along the bond interfaces. Furthermore,visual inspection of these samples showed microstructure changes alongthe electrode interfaces, but at very shallow, removable depths (e.g.,less than 0.020 inch (0.5 mm) in depth).

While this invention has been described as having exemplary designs, thepresent invention can be further modified within the spirit and scope ofthis disclosure. This application is therefore intended to cover anyvariations, uses, or adaptations of the invention using its generalprinciples. Further, this application is intended to cover suchdepartures from the present disclosure as come within known or customarypractice in the art to which this invention pertains and which fallwithin the limits of the appended claims.

What is claimed is:
 1. A method of manufacturing an orthopedicprosthesis comprising the steps of: providing a metal substrate;providing a porous metal layer having a thickness; positioning theporous metal layer against the metal substrate to form an interfacebetween the porous metal layer and the metal substrate; directlycontacting the porous metal layer with a textured contacting surfaceintegral with an electrode during welding, wherein the texturedcontacting surface of the electrode approximates surface features of theporous metal layer to be directly contacted by the electrode duringwelding; and directing an electrical current to the interface betweenthe porous metal layer and the metal substrate to resistance weld theporous metal layer to the metal substrate Ode maintaining the thicknessof the porous metal layer, the current traveling from the electrode,through the porous metal layer, and toward the metal substrate.
 2. Themethod of claim 1, further comprising the step of applying a weldpressure to hold the porous metal layer against the substrate, whereinthe weld pressure is sufficiently low to avoid deforming the porousmetal layer.
 3. The method of claim 2, wherein the weld pressure is lessthan 3,000 psi (20.7 MPa).
 4. The method of claim 1, wherein theelectrical current comprises a pulsed electrical current, the pulsedelectrical current includes at least 10 pulses.
 5. The method of claim4, wherein the directing step comprises directing each pulse of thepulsed electrical current to the orthopedic prosthesis at a currentdensity of at least 75 kA/in² (11.6 kA/cm²).
 6. The method of claim 4,wherein pulses of the pulsed electrical current are separated by acooling time, and the cooling time comprises less than 1 second.
 7. Themethod of claim 1, wherein the directing step is performed in acontrolled atmosphere having an oxygen concentration less than about 10ppm.
 8. The method of claim 1, wherein the directing step bonds theporous metal layer and the metal substrate together at a tensilestrength of 2,900 psi (20.0 MPa) or more.
 9. The method of claim 2,further comprising the step of: applying a forge pressure to theorthopedic prosthesis after the directing step.
 10. The method of claim9, wherein the forge pressure is less than the weld pressure.
 11. Themethod of claim 1, wherein the porous metal layer includes reticulatedvitreous carbon foam.
 12. The method of claim 11, further comprisingcoating the reticulated vitreous carbon foam, wherein the coating stepcomprises a chemical vapor deposition step to coat the outer surface ofthe porous structure with metal and to infiltrate the porous structurewith metal.
 13. The method of claim 1, wherein the textured contactingsurface of the electrode includes knurling.
 14. The method of claim 13,wherein the textured contacting surface of the electrode includespyramid-shaped projections.
 15. The method of claim 1, wherein: theporous metal layer is previously manufactured and has a porosity of 55%to 90%, and positioning of the porous metal layer generates a netcontact area of 15% to 25% between the previously manufactured porousmetal layer and the metal substrate at said interface; and the methodfurther comprises applying a weld pressure holding the previouslymanufactured porous metal layer against the metal substrate, whereinsaid weld pressure is at least 100 psi (0.7 MPa) but less than 3,000 psi(20.7 MPa), and wherein said net contact area is maintained despiteapplying said weld pressure.
 16. The method of claim 14, wherein thepyramid-shaped projections include projections of different sizes. 17.The method of claim 14, wherein the knurling includes pyramid-shapedprojections of 45 degree and 0.040 pitch geometries.